Coatings for cutting tools

ABSTRACT

A cutting tool insert comprises a hard metal substrate having at least two wear-resistant coatings including an exterior ceramic coating and a coating under the ceramic coating being a metal carbonitride having a nitrogen to carbon atomic ratio between 0.7 and 0.95 which causes the metal carbonitride to form projections into the ceramic coating improving adherence and crater resistance of the ceramic coating. Also disclosed is a cutting tool insert including a hard substrate and at least first and second coatings on at least a portion of said substrate. The first coating is of at least about 2 microns, is in contact with the substrate, and includes at least one of a metal carbide, a metal nitride, and a metal carbonitride of a metal selected from the group consisting of zirconium and hafnium. The second coating may include at least one of a metal carbide, a metal nitride, and a metal oxide of a metal selected from groups IIIA, IVB, VB, and VIB of the periodic table.

FIELD OF THE INVENTION

The present invention relates to the field of cutting tools andparticularly to coatings for ceramic coated hard metal cutting toolinserts used for cutting, milling, drilling and other applications suchas boring, trepanning, threading and grooving.

BACKGROUND OF THE INVENTION

Coatings improve the performance of cutting tools, especially ceramic oroxide coatings on carbide or hard metal cutting tools. Ever sincecarbide cutting tool inserts have been ceramic coated with, for example,aluminum oxide (Al₂O₃), there has been a continuing effort to improvethe adherence of the coating to the substrate. When the first aluminumoxide coating was applied directly to a substrate of the carbide or hardmetal type, the oxygen in the aluminum oxide reacted with the substratewhich reduced the adherence.

It has been known to improve the properties of tool inserts made from asintered hard metal substrate (metallic carbide bonded with a bindermetal) by applying a wear-resistant carbide layer. See UK Patent Nos.1,291,387 and 1,291,388 which disclose methods of applying a carbidecoating with improved adherence; specifically, controlling thecomposition of the gas used for deposition of the carbide so that adecarburized zone was formed in the sintered hard metal at the interfacewith the wear-resistant carbide. The decarburized zone known as an etalayer, however, tends to be hard and brittle, resulting in breakage. Ithas also been known to apply a ceramic or oxide wear-resistant coating(usually aluminum oxide) upon the sintered metal substrate. However, asalready explained, the oxide layer directly upon the sintered metal bodymay disrupt the sintered metal morphology and binding ability. A numberof patents have disclosed the use of an intermediate layer of carbides,carbonitrides and/or nitrides. See U.S. Pat. Nos. 4,399,168 and4,619,866. An intermediate titanium carbide (TiC) layer improvedtoughness but still an eta layer existed limiting the application of thecoated tool inserts to finishing cuts. A layer of titanium nitride (TiN)applied before the TiC layer eliminated the eta layer but toughness wasstill less than required. See U.S. Pat. No. 4,497,874. Intermediatelayers of titanium carbonitride (TiCN) in place of the TiC intermediatelayer have been proposed. See U.S. Pat. Nos. 4,619,866 and 4,399,168. Athin surface oxidized bonding layer comprising a carbide or oxycarbideof at least one of tantalum, niobium and vanadium between the hard metalsubstrate and the outer oxide wear layer has been proposed. See U.S.Pat. No. 4,490,191.

The ceramic coating (Al₂O₃) does not adhere well enough to the TiC andmany TiCN intermediate coatings when used to enhance-the adhesion of thecoating to the cemented carbide substrate. Due to thermal expansiondifferences, there is a tendency to delaminate. With the stress causedby the thermal expansion difference, coatings tend to performinconsistently. These intermediate coatings are mostly characterized bya straight line interface between the intermediate coating and the oxidecoating as shown in FIG. 1. This results in a weak bond. Adhesion may beincreased some by making the substrate rough but the projectionsprovided by the roughening are spaced too far apart to performconsistently.

Another problem experienced with carbide and hard metal cutting tools isthe frequent failure of those tools due to thermal shock. The insertsbecome very hot during cutting and then cool upon application ofcoolants or when disposed outside the cut. Cycles of heating and coolingresult in steep temperature gradients within the inserts, and theaccompanying stresses may cause cracks in the inserts that initiatefractures and reduce tool life. Thus, coatings that reduce theoccurrence of fractures from thermal shock may considerably enhance toollife.

With the coatings, according to the present invention, increased wearresistance as well as adhesion strength are provided in ceramic coatingson hard metal cutting tools. According to another aspect of theinvention, coatings are provided that reduce thermal shock experiencedby carbide and hard metal cutting tool inserts.

SUMMARY OF THE INVENTION

Briefly, according to this invention, there is provided a cutting toolinsert comprising a hard metal substrate having at least twowear-resistant coatings. One of the coatings is a ceramic coating. Anintermediate coating under the ceramic coating is comprised ofcarbonitride having a nitrogen to carbon atomic ratio between about 0.7and about 0.95 whereby the carbonitride coating forms fingersinterlocking the ceramic coating, thus improving the adherence andfatigue strength of the ceramic coating. Preferably, the nitrogen tocarbon atomic ratio in the carbonitride coating lies between about 0.75and 0.95 as determined by X-ray diffraction. The cutting tool insertalso may include an additional coating, deposited on the substrate, thatis a layer of at least about 2 microns, and preferably at least about 2up to about 5 microns, in thickness and comprises at least one of ametal carbide, a metal nitride, or a metal carbonitride of a metalselected from zirconium and hafnium.

According to one embodiment of this invention, the hard metal cuttingtool insert has two intermediate coatings between the hard metalsubstrate and the aluminum oxide surface coating. The coating adjacentthe substrate is a 1 to 4 micron layer of titanium nitride. The coatingover the titanium nitride layer is a 2 to 4 micron thick titaniumcarbonitride layer and the aluminum oxide coating is a 1 to 10 micronlayer.

According to the preferred embodiment, the hard metal substrate of thecutting tool insert has four coatings as follows: a 2 micron titaniumnitride interior coating, a 3 micron titanium carbonitride intermediatecoating, a 6 micron aluminum oxide intermediate coating, and a 2 micronTi (C, N), i.e., TiC, TiN, TiC_(x)N_(y) exterior coating.

Titanium is not the only suitable metal for use in the carbonitridecoating. The metal may be comprised of, in addition to titanium,zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenumand tungsten.

The cutting tool insert substrate, according to this invention,typically comprises 3% to 30% of a binder metal from the iron groupincluding, in addition to iron, nickel and cobalt and mixtures thereofand between 70% and 97% of a carbide selected from the group tungstencarbide, titanium carbide, tantalum carbide, niobium carbide, molybdenumcarbide, zirconium carbide and hafnium carbide. In addition to carbides,the cutting tool insert substrate may also include nitrides.

According to a preferred embodiment, the cutting tool insert substratehas a binder phase enriched surface layer, that is, a surface layerenriched with a higher percentage of cobalt or other binder metal.

Briefly, according to this invention, there is provided a method ofmaking a coated cutting tool insert having a wear-resistant coatingcomprising the steps of depositing a metal carbonitride coating having anitrogen to carbon atomic ratio between about 0.7 and about 0.95 byadjusting the reactants used for chemical vapor deposition of saidcoating and depositing a ceramic coating directly over said carbonitridecoating whereby said carbonitride coating and ceramic coating haveinterlocking microscopic fingers.

According to another aspect of the invention, there is provided acutting tool insert including a hard substrate and a plurality ofcoatings on at least a portion of the substrate. The substrate may beany type suitable for use as a cutting tool insert and may be, forexample, a cemented carbide as described above. The plurality ofcoatings includes at least a first and a second coating. The firstcoating is a layer at least about 2 microns and preferably about 2 toabout 5 microns in thickness deposited on the substrate and includes atleast one a metal carbide, a metal nitride, or a metal carbonitride of ametal selected from zirconium and hafnium. Preferably, the first coatingis a layer of zirconium nitride or hafnium nitride. The second coatingis a layer including at least one of a metal carbide, a metal nitride,or a metal oxide of a metal selected from groups IIIA (B, Al, Ga), IVB(Ti, Zr, Hf), VB (V, Nb, Ta), and VIB (Cr, Mo, W) of the periodic table.One or more additional layers optionally may be provided intermediatethe first and second coatings and also may be deposited exterior to thesecond coating. Thus, for example, the plurality of coatings may includea reinforcing coating, as described herein, provided intermediate thefirst and second coatings. The intermediate coating contacts andenhances adhesion of the second coating. More particularly, theintermediate coating may be a layer including a metal carbonitride that,as described herein, has a nitrogen to carbon atomic ratio that resultsin superior adherence of the second coating due to the development ofinterlocking fingers between the second coating and the intermediatecoating.

Designations such as “first”, “second”, and “third” are used herein toidentify individual coatings or layers only and, in the presentdescription and the attached claims, do not necessarily refer to theordering of the layers or coatings or their sequence of application onthe substrate. Thus, for example, a “first” coating or layer is notnecessarily in contact with or immediately adjacent a “second” coatingor layer, and a “third” coating or layer, as well as additional coatingsor layers, may be deposited intermediate the “first” and “second”coatings or layers.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and other objects and advantages of this invention willbecome clear from the following detailed description made with referenceto the drawings in which:

FIG. 1 is a photomicrograph of a polished section of a hard metalcutting tool insert having an oxide coating and an intermediate coatingaccording to the prior art;

FIGS. 2-4 are photomicrographs of polished sections of hard metalcutting tool inserts, according to this invention, having anintermediate coating and an oxide coating;

FIGS. 5 and 6 are graphs showing the total number of thermal cracksdeveloped to failure along the edge of inserts constructed according tothe present invention with various single or multiple-layer coatingsduring dry milling (FIG. 5) and wet milling (FIG. 6) of rectangularsteel stock;

FIG. 7 is a graph showing the total number of thermal cracks developedalong the edge of inserts constructed according to the present inventionwith coatings including hafnium nitride and aluminum oxide layers duringmilling of rectangular steel stock; and

FIG. 8 is a photomicrograph of a polished section of a hard metalcutting tool, according to this invention, having a coating including ahafnium nitride innermost layer, an Al₂O₃ exterior layer, and a titaniumcarbonitride intermediate layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to an aspect of this invention, hard metal cutting tools witha ceramic or oxide wear-resistant coating have a novel reinforcingintermediate coating. The hard metal substrate has a thin metal nitridecoating overlaid with a titanium carbonitride coating. Thewear-resistant ceramic coating overlays the metal carbonitride coating.The metal carbonitride intermediate layer is provided with a nitrogen tocarbon atomic ratio that results in superior adherence of the oxidecoating due to the development of interlocking fingers between the oxidecoating and the metal carbonitride coating.

A test was devised to quantitatively evaluate the performance of ceramiccoated hard metal cutting tool inserts. The test is performed on aturning machine. The stock is a cylindrical bar having a diametergreater than about 4 inches. The bar has four axial slots ¾ inch wideand 1½ inches deep extending the length of the bar. The bar is mediumcarbon steel AISI-SAE 1045 having a hardness of 25-30 HRC. The tools tobe tested were used to reduce the diameter of the stock as follows. FeedRate Speed (inches per Depth of Cut (surface feet per revolution or IPR)(inches) minute or SFM) .020 .050 500

It should be apparent that four times per revolution of the stock, thecutting tool insert impacts the edge of a slot. The cutting tool insertis run until it breaks through the coating or another failure isobserved. Failures were observed in the following described test andwere of the fretting type which is a precursor to the greater wear andcutting failure type.

In the following examples, the nitrogen to carbon atomic ratio in thetitanium carbonitride intermediate layer or coating was determined byuse of X-ray diffraction to first detect the lattice spacing of thecarbonitride layer and then to calculate the atomic ratio of nitrogen tocarbon or the atomic percentage of nitrogen based upon nitrogen andcarbon. The lattice spacing of titanium carbide is known to be 1.53Angstroms and the lattice spacing for titanium nitride is known to be1.5 Angstroms. The range or difference is 0.03 Angstroms. Thus, atitanium carbonitride layer found to have a lattice spacing of 1.5073Angstroms is 0.0227 Angstroms between the spacing for titanium nitrideand titanium carbide. Hence, the atomic ratio of nitrogen to carbon is0.0227 divided by 0.03 or 75.7% nitrogen based on total carbon andnitrogen in the carbonitride layer.

EXAMPLE I COMPARATIVE EXAMPLE

A tungsten carbide based substrate (94% tungsten carbide, 6% cobalt) ofK20 material (K20 is a designation of the type of hard cutting materialfor machining as set forth in ISO Standard ISO513:1991(E) classifiedaccording to the materials and working conditions for which the hardmetal cutting material can appropriately be used) was coated accordingto well-known procedures in a Bemex Programmat 250 coating furnace. Thecoating process known as chemical vapor deposition (CVD) was used wheregasses and liquids (converted to gas) are passed over substrates to becoated at 800° to 1,100° C. and reduced pressures from 50 to 900 mbar.The reactions used to coat the hard metal substrate were as follows:CVD of TiN−uses H₂+N₂+Titanium Tetrachloride (TiCl₄)CVD of TiCN−uses H₂+N₂+TiCl₄+Acetonitrile (CH₃CN) or CH₄CVD of Al₂O₃−uses H₂+HCl+Aluminum Chloride (AlCl₃)+CO₂+H₂S

The essential coating periods and atmospheres used to apply the titaniumnitride layer, the titanium carbonitride layer and the oxide layer areset forth in the following Tables I, II and III. The gas reactants, theproduct of the AlCl₃ reactor and the liquid reactions are introduced tothe furnace. TABLE I Run Time Millibar Reactor ° C. Coating MinutesPressure Reactor Temp. TiN 60 160 920 TiCN 420 60 870 Al₂O₃ 270 60 1005

TABLE II Gas Reactants Liter/Minute Coating H₂ N₂ CO₂ CH₄ HCl H₂S TiN 149 TiCN 14 8 Al₂O₃ 11 0.6 .20 0.050

TABLE III AlCl₃ Gas Liquid reactants Generator ml/min l/min CH₃CN TiCl₄Coating H₂ HCl Liquid Liquid TiN 2.1 TiCN 125 2.4 Al₂O₃ 1.9 0.8

X-ray analysis of the titanium carbonitride layer demonstrated a latticespacing of 1.516 Angstroms which, based on the analysis explained above,represents a nitrogen to carbon atomic ratio of 14:30 or a nitrogencontent of 46.7% based on the total carbon and nitrogen in thecarbonitride layer. The coated tool according to this example wassubmitted to the above-described machining test. After only 14.5seconds, fretting was displayed.

FIG. 1 is a photomicrograph of a polished section showing the layers orcoatings over the substrate. Notice that the interface between thetitanium carbonitride and oxide layer is almost a straight line, thatis, there are no interlocking fingers.

EXAMPLE II

A coating, according to this invention, was prepared on a tungstencarbide based substrate in the coating furnace above described with thecoating periods and atmospheres as described in Tables IV, V and VI.TABLE IV Run Time Millibar Reactor ° C. Reactor Coating Minutes PressureTemp. TiN 60 160 920 TiCN 240 80 1005 Al₂O₃ 540 60 1005

TABLE V Gas Reactants Liter/Minute Coating H₂ N₂ CO₂ CH₄ HCl H₂S TiN 149 TiCN 11.3 8 0.6 Al₂O₃ 11 0.6 0.2 .050

TABLE VI AlCl₃ Gas Generator Liquid Reactants l/min ml/min Coating H₂HCl CH₃CN Liquid TiCl₄ Liquid TiN 2.1 TiCN 0.9 Al₂O₃ 1.9 0.8

Tables IV, V and VI, in addition to showing the run times, reactionpressures and temperatures, show the rate of gas reactants, aluminumchloride generator reactants and the liquid reactants. The gas reactantsintroduced into the aluminum chloride generator flow over aluminum metalchips producing a quantity of aluminum chloride which is passed into thecoating furnace.

X-ray analysis of the titanium carbonitride layer demonstrated a latticespacing of 1.5073 which, based on the analysis explained above,represents a nitrogen to carbon ratio of 23:30 or a nitrogen content of75.7% based upon the total carbon and nitrogen in the carbonitridelayer.

The coated tool insert was submitted to the above-described machiningtest. The cutting test showed no fretting at 180 seconds. FIG. 2 is aphotomicrograph of a polished section showing the layers of coating overthe substrate. The photomicrograph illustrates fingers or anchors of thetitanium carbonitride layer penetrating the oxide layer and anchoring itin place.

EXAMPLE III

Example III was prepared the same as Example II except the nitrogen waslower in the coating furnace during the deposition of the carbonitridelayer. The lattice spacing in the titanium carbonitride layer was foundto be 1.509 which represents a nitrogen to carbon atomic ratio of 21:30or a nitrogen content of 70%.

In the machining test, fretting was displayed only after a 5-inch cutlength (estimated 40 to 50 seconds). The microstructure of Example IIshown in FIG. 3 anchors between the oxide and the titanium carbonitridelayers are displayed but are very minor.

EXAMPLE IV

Example IV was prepared the same as Example II except with increasednitrogen flow. The lattice spacing of the titanium carbonitride layerwas 1.503 Angstroms which represents a nitrogen to carbon atomic ratioof 27:30 or 90% nitrogen. In the machining test, the tool insertdisplayed no fretting after 120 seconds. The microstructure of ExampleIV is shown in FIG. 4 and illustrates prominent fingers or anchorsextending between the carbonitride layer and the oxide layer.

EXAMPLE V

In the following example, tool inserts coated according to thisinvention were machine tested with the following cutting conditions. Thestock was 3,000 gray cast iron 200 BHN. The tools tested were used toreduce the diameter of the stock as follows. Feed Rate Speed (inches perDepth of Cut (surface feet per revolution or IPR) (inches) minute orSFM) .022 .100 950

Two steel inserts, according to this invention, ran 108 pieces per edge.By comparison, a C-5 alumina coated tool insert ran 50 pieces per edge.The tool inserts, according to this invention, were a 100% improvement.

EXAMPLE VI

In the following example, the stock for the machining test was ARMAsteel 250 BHN. The machining conditions were as follows. Feed Rate Speed(inches per Depth of Cut (surface feet per revolution or IPR) (inches)minute or SFM) .010 .100 1,200

Using the tool inserts, according to this invention, 170 pieces per edgewere run. By comparison, with C-5 alumina coated tool inserts, 85 piecesper edge were run. The tool inserts, according to this invention, were a100% improvement.

Coatings Reducing the Occurrence of Thermal Cracking

According to the present invention, there also is provided cutting toolinserts having a coating that reduces the occurrence of cracks resultingfrom thermal shock during milling and other machining operations. Thecoating is applied directly on the insert's hard metal substrate andincludes one or more metal carbides, one or more metal nitrides, and/orone or more metal carbonitrides of hafnium and zirconium. Restated, thecoating may include one or more of the materials zirconium carbide,zirconium nitride, zirconium carbonitride, hafnium carbide, hafniumnitride, and hafnium carbonitride. Hafnium carbonitride and zirconiumcarbonitride, for example, refer to materials including HfC_(x)N_(y) andZRC_(x)N_(y), respectively, wherein 0.7<(x+y)<1.3.

The coating of the invention is applied as the innermost layer of amulti-layer coating wherein the innermost layer inhibits the formationof thermal cracking. The additional layers of the multi-layer coatingoverlay the innermost layer and provide additional advantageousproperties such as, for example, enhanced wear resistance and/or craterresistance. The innermost layer preferably is 2 to 5 microns inthickness so as to reduce the occurrence of thermal cracks duringcutting. The additional, overlying layers may include a ceramic or oxidewear-resistant layer, in which case the additional layers also mayinclude the novel reinforcing intermediate layer of the presentinvention comprising a carbonitride having a nitrogen to carbon atomicratio, preferably between 0.7 and 0.95, that results in the formation ofinterlocking fingers between the ceramic or oxide wear-resistant coatingand the carbonitride coating. Thus, the present invention includescoatings combining the coating of the invention that inhibits thermalcracking along with the reinforcing coating of the present inventionthat increases the adherence and crater-resistance of overlyingwear-resistant coatings. Accordingly, embodiments of cutting tool insertwithin the present invention may include:

-   -   a hard metal substrate;    -   an innermost layer deposited directly on the substrate and that        includes at least one of hafnium nitride and zirconium nitride;    -   an exterior layer including a wear-resistant ceramic or oxide        material; and    -   an reinforcing layer intermediate the innermost and exterior        layers and in contact with the exterior layer, wherein the        intermediate layer is a metal carbonitride having a nitrogen to        carbon atomic ratio between 0.7 and 0.95 and wherein projections        of the metal carbonitride form in the exterior layer and enhance        the adherence and crater resistance of the exterior layer.

As described in detail above, the reinforcing intermediate metalcarbonitride layer more preferably has a nitrogen to carbon atomic ratioof 0.75 to 0.95 as determined by x-ray diffraction, and it also ispreferred that the carbonitride layer have a nitrogen content of 70% to90% based upon the total nitrogen and carbon content of the reinforcinglayer.

Other possible coating layers that may overlay the thermal crackinhibiting coating of the present invention include wear-resistantlayers composed of one or more of carbides, nitrides, borides, andoxides of metals within groups IIIA, IVB, VB, and VIB of the periodictable. Preferably, such additional layers are individually about 2 toabout 10 microns in thickness.

Several cutting tool inserts according to the present inventionincluding a layer of a metal nitride or metal carbonitride applied tothe surface of a cemented carbide substrate were prepared and evaluatedin the following examples.

EXAMPLE VII

Several cutting tool inserts of style SEKN42AF4B composed of H-91 gradecemented carbide material were coated with the single or multiple-layercoatings indicated in Table VII. The rightmost indicated layer (eitherhafnium nitride or titanium nitride) is the layer that was applieddirectly on the surface of the substrate. The additional layers werethen applied in the indicated sequence from right to left. TABLE VIICoating Thickness (μm) Coating TiCN TiCN No. Substrate TiN Al₂0₃ (NL)(MT) TiN HfN Total #1 H-91 2.5 3.4 6.4 1.5 13.8 #2 H-91 3.9 <1 3.3 7.2#3 H-91 1.5 2.1 Trace 3.6 #4 H-91 2.9 2.9 #5 H-91 1.2 2.1 2.7 0.5 6.5

Inserts of H-91 grade material are available from Stellram, LaVergne,Tennessee, and are comprised of 88.5 weight percent tungsten carbide,11.0 weight percent cobalt, and 0.5 weight percent of a mixture oftitanium carbide, tantalum carbide, and niobium carbide. The H-91material exhibits a hardness of 89.7 HRA, 14.40 g/cc density, and atransverse rupture strength of approximately 389,000 psi.

As indicated in Table VII, the titanium carbonitride layers in coatingnos. 1, 2, 3, and 5 were applied either as a reinforcing coating(designated NL) or as a moderate temperature coating (MT). Coating no. 1includes both titanium carbonitride coating types.

The coatings were applied to the inserts using well known CVD techniquesthat may be replicated by those of ordinary skill without undue effort.Hafnium nitride layers were deposited in connection with coating nos. 2and 4 as follows. A Bemex 250 CVD coating furnace was prepared byintroducing into the coating chamber of the furnace a 10 liters/minuteflow of nitrogen gas. Hafnium metal sponge was placed in the generatorchamber of the coating furnace and, concurrent with the nitrogen flow inthe coating chamber, a flow of 5 liters/minute of nitrogen gas wasintroduced into generator chamber. A 200 mBar nitrogen gas pressure wasestablished within both the coating and generator chambers. The coatingchamber was then heated to 1080° C., while the generator chamber washeated to 425° C. When those temperatures were reached, the pressurewithin each chamber was allowed to increase to 800 mBar. Whilemaintaining the pressure at 800 mBar, nitrogen flow into the coatingchamber was increased to 10.8 liter/minute, and hydrogen gas wasintroduced into that chamber at 6.8 liters/minute. The original 5liter/minute flow of nitrogen through the generator chamber wasmaintained. Each chamber was allowed to stabilize for about 2 minutes. Aflow of chlorine gas was then introduced into the generator chamberconcurrently with the 5 liters/minute flow of nitrogen gas. Theconcurrent flow of gases produced hafnium chlorides which, when combinedwith the additional gases flowing within the furnace chamber for a12-hour period, deposited a coating of hafnium nitride on the surface ofthe H-91 substrate.

After the 12-hour coating time, the chlorine gas through the generatorchamber was turned off, and the generator and furnace chambers werepurged of gases for 20 minutes with all other conditions remaining thesame. After 20 minutes, the flows of nitrogen through both the generatorand coating chambers were shut off and the hydrogen flow into thegenerator chamber was increased to 10 liters/minute. The generatorchamber temperature was lowered to room temperature, and the coatingchamber temperature was allowed to ramp down to 1015° C. at a rate of0.5° C./minute. The reduction in temperatures took approximately 130minutes. The generator chamber charged with hafnium sponge was not usedduring any subsequent coating steps in inserts having coatings withadditional layers.

The titanium nitride layers of coatings #1, #3, and #5 were depositedusing the coating conditions provided in Example I, Tables I-III for runtimes as appropriate. The Al₂O₃ layers of the coatings were deposited byCVD by first heating the coating chamber to 1015° C., and then the gasflows, pressures, and times shown in Tables I-III were used. The NLtitanium carbonitride layers of the coatings were deposited by firstheating the insert to 1015° C. and then using the general conditionsshown in Table VIII. The NL titanium carbonitride formed projectionsinto the immediately overlying layer and enhanced the adhesion of thatlayer to the insert. The MT titanium carbonitride coatings weredeposited by first heating the insert to a moderate temperature of 870°C. and using the general conditions shown in Tables I-III where a flowof acetonitrile is substituted for methane. The substitution ofacetonitrile for methane results in a lower reaction rate and,therefore, the MT titanium carbonitride coatings do not form anchoringprojections into the immediately overlying layer of the coatings. TABLEVIII Total time: 180 minutes H₂ flow: 11.3 l/min. N₂ flow: 10 l/min. CH₄flow: 0.6 l/min. TiCl₄ flow: 0.9 ml/min. Chamber pressure: 300 mBar

The resistance of the coated inserts to thermal cracking was evaluatedunder both wet and dry conditions using the inserts to reduce thesurface of 3″X12″X6″ rectangular stock of 33-35 HRC AISI type 4150 steelunder the following conditions. Feed Rate Depth of Speed (inches per Cut(surface feet per revolution or IAR) (inches) minute or SFM) 0.15 .100800

FIGS. 5 and 6 are graphs showing the total number of thermal cracksdeveloped along the edge of each of the coated inserts after each passuntil failure. Failure was defined as the condition in which thermalcracks connect and cause the insert edge to chip or deform during thecut. FIG. 5 shows data derived under dry milling conditions, while FIG.6 shows data derived under wet milling conditions. Certain observationsmade during milling are indicated on the figures adjacent the passduring which the particular condition was observed.

The test data in FIGS. 5 and 6 shows that the inserts coated with aninner layer of hafnium nitride and an exterior layer of aluminum oxide(coating no. 2) performed best in both wet and dry milling, resistingformation of thermal cracks and breaking. By comparing the performanceof the insert having coating no. 2 (3.3 μHFN, <1 μTiCN, and 3.9 μAl₂O₃)with that of the insert having coating no. 4 (2.9 μHFN only) under bothwet and dry milling conditions, the reduction in thermal crackingachieved by addition of an aluminum oxide overlayer is seen. The inserthaving coating no. 3 (trace TiN, 2.1 μTiCN, and 1.5 μAl₂O₃) exhibitedthe least resistance to thermal cracking. The favorable performance ofcoating no. 2 is attributed, at least in part, to the reinforcingintermediate titanium carbonitride layer, which formed interlockingprojections into and enhanced the adhesion and crater resistance of theoverlying aluminum oxide layer.

It is noted that the insert sample coated only with hafnium nitride(coating no. 4) performed well (i.e., resisted thermal cracking) untilthe coating wore off. The superior performance of coating no. 2indicates that an overlying wear-resistant layer applied to the hafniumnitride layer of coating no. 4 would further enhance the thermal crackinhibiting effect of that the nitride layer. Depositing the ceramiclayer onto the metal nitride layer also augments the total thickness ofthe coating on the insert. Total coating thicknesses of 8 microns ormore may be achieved by applying a ceramic layer and, possibly, one ormore intermediate metal carbonitride layers onto the insert in additionto the innermost metal nitride layer.

The use of metal nitride and/or metal carbonitride layers to inhibitthermal cracking provides certain distinct advantages over conventionallayers deposited by physical vapor deposition (PVD) used to reducethermal cracking. For example, metal nitride and metal carbonitridelayers may be applied by CVD, which allows for the deposition of thickerlayers than by PVD. Also, nitrides and carbonitrides of hafnium andzirconium, for example, are chemical vapor deposited at relatively hightemperature and will better adhere to the substrate relative to coatingsapplied at lower temperatures. The thermal expansion of a layer of anyof the nitrides and carbonitrides of hafnium and zirconium is close tothat of a cemented carbide substrate and, therefore, spalling of themetal nitride or carbonitride layer is reduced. Nitrides andcarbonitrides of hafnium and zirconium also have thermal expansioncoefficients close to that of overlying titanium carbonitride and/oraluminum oxide layers, thereby reducing spalling of those overlyinglayers. In addition, the free energies of formation of nitrides andcarbonitrides of hafnium and zirconium are low and there is no tendencyfor eta layer formation (a hard and brittle layer generated in thesubstrate that reduces toughness).

EXAMPLE VIII

Additional experiments were performed to assess the performance ofcoatings of the present invention including a metal nitride innermostlayer, a metal oxide exterior layer, and, optionally, a metalcarbonitride intermediate layer. Milling inserts (style SEKN-42AF4Bcomposed of H-91 grade cemented carbide material) were prepared with thecoatings indicated in Table IX by CVD using well known depositiontechniques as generally described above. Coating no. 6 includes atitanium carbonitride intermediate layer, which is absent in coating no.7. The coated inserts were then used to reduce the top of 33-35 HRC AISI4150 steel 3″X 5″X 12″ rectangular stock using the machining conditionsapplied in Example VII above. TABLE IX Coating Thickness (microns)Coating No. Substrate Al₂O₃ TiCN HfN Total #6 H-91 4.5 <1 3.6 8.1 #7H-91 4.5 none 2.3 6.8

FIG. 7 is a graph showing the total number of thermal cracks developedalong the edge of the coated inserts under either wet or dry millingconditions until failure. The failure condition is as described inconnection with Example VII. Based on the results shown in FIG. 7, thepresence of the metal carbonitride intermediate layer is preferred underwet milling conditions as it enhances the adhesion of the exterior metaloxide layer. FIG. 8 is a photomicrograph of a section through coatingno. 6 showing the interlock of coatings at the interface of the titaniumcarbonitride and Al₂O₃ layers. The insert having coating no. 7 insert,which lacked the metal carbonitride intermediate layer, experiencedfretting of the aluminum oxide exterior layer on the first pass duringwet milling. The data of FIG. 7 also indicates that an innermost layerof hafnium nitride or zirconium nitride preferably is at least about 4microns thick to enhance resistance to thermal cracking.

Based on the improved thermal crack resistance and the wear resistanceachieved with a coating including an innermost hafnium nitride orzirconium nitride layer, a ceramic or oxide exterior layer, and,optionally, an intermediate metal carbonitride layer disposed in contactwith the exterior layer, a cutting tool insert constructed as followswould exhibit particularly advantageous resistance to wear, cratering,and thermal cracking:

-   -   (i) a hard metal or cemented carbide substrate such as, for        example, a cemented carbide substrate including 3 to 30 weight        percent of one or more binder metals from the iron group        (including iron, nickel, and cobalt) and 70 to 97 weight percent        of one or more metal carbides and/or one or more metal nitrides        of tungsten, titanium, tantalum, niobium, molybdenum, zirconium,        or hafnium;    -   (ii) a 2 to 5 micron layer, applied directly on the substrate,        of at least one metal nitride or metal carbonitride of zirconium        or hafnium;    -   (iii) a 1 to 10 micron layer, exterior to layer (ii), of a        wear-resistant ceramic or oxide, such as, for example, aluminum        oxide;    -   (iv) optionally, a 2 to 6 micron layer of a metal carbonitride        (for example, titanium carbonitride) deposited immediately under        and in contact with the ceramic or oxide layer; and    -   (v) optionally, a 1 to 4 micron layer of a metal nitride (for        example, titanium nitride) applied exterior to the ceramic        layer.

A more specific construction of a coated cutting tool insert accordingto the present invention may include the following in the indicatedsequence:

-   -   (i) a cemented carbide substrate;    -   (ii) a 4 micron hafnium nitride innermost layer;    -   (iii) a 3-5 micron titanium carbonitride layer;    -   (iv) a 2-4 micron aluminum oxide layer; and    -   (v) a 1 micron titanium nitride exterior layer.

The metal nitride innermost layer of the above embodiments of theinvention reduces the occurrence of thermal cracking. The metal nitridelayer by itself, however, does not have substantial wear resistance. Toenhance wear resistance, the exterior ceramic or oxide layer is alsoapplied. To enhance the ceramic or oxide layer's adhesion and resistanceto cratering, the metal carbonitride intermediate layer also may beprovided. According to the present invention, the nitrogen to carbonatomic ratio of the metal carbonitride intermediate layer preferably isadjusted to the range 0.7 to 0.95, and preferably 0.7 to 0.9, to promotethe formation of projections of the intermediate layer into the ceramicor oxide layer.

Having thus described our invention with the detail and particularityrequired by the Patent Laws, what is desired protected by Letters Patentis set forth in the following claims.

1. A cutting tool insert comprising a hard substrate and a plurality ofcoatings on at least a portion of said substrate, said plurality ofcoatings including: a first coating of at least 2 microns deposited onsaid substrate, said first coating comprising at least one of a metalcarbide, a metal nitride, and a metal carbonitride of a metal selectedfrom the group consisting of zirconium and hafnium; and a second coatingcomprising at least one of a metal carbide, metal nitride, and metaloxide of a metal selected from groups IIIA, IVB, VB, and VIB of theperiodic table.
 2. The cutting tool insert of claim 1, wherein saidfirst coating is at least 2 microns up to 5 microns.
 3. The cutting toolinsert of claim 1, wherein: said first coating is selected from thegroup consisting of zirconium nitride and hafnium nitride; and saidsecond coating is one of aluminum oxide and titanium nitride and is 1 to10 microns thick.
 4. The cutting tool insert of claim 1, furthercomprising: a third coating that is a coating of a metal carbonitride 2to 6 microns thick, said third coating intermediate said first coatingand said second coating and in contact with said second coating.
 5. Thecutting tool insert of claim 4, wherein said metal carbonitride of saidthird coating has a nitrogen to carbon atomic ratio between 0.7 and 0.95which causes said metal carbonitride of said third coating to formprojections into said second coating to thereby improve adherence andcrater resistance of said second coating.
 6. The cutting tool insert ofclaim 4, wherein: said first coating is a coating of hafnium nitride atleast 4 microns thick; said second coating is a coating of aluminumoxide 2 to 4 microns thick; and said third coating is a coating oftitanium carbonitride 3 to 4 microns thick.
 7. The cutting tool insertof claim 6, wherein said plurality of coatings further comprises: afourth coating that is a coating of titanium nitride at least 1 micronthick, said fourth coating overlying said second coating.
 8. A cuttingtool insert comprising a hard substrate and a plurality of coatings onat least a portion of said substrate, said plurality of coatingsincluding: a first coating deposited on said substrate and comprising atleast one of a metal carbide, a metal nitride, and a metal carbonitride,wherein said metal is selected from the group consisting of zirconiumand hafnium; and a second coating comprising a ceramic; and a thirdcoating, intermediate said first coating and said second coating and incontact with said second coating, said third coating comprising a metalcarbonitride having a nitrogen to carbon atomic ratio between 0.7 and0.95 which causes said metal carbonitride to form projections into saidceramic coating to thereby improve adherence and crater resistance ofsaid second coating.
 9. The cutting tool insert of claim 8, wherein saidfirst coating is 2 to 5 microns thick.
 10. The cutting tool insert ofclaim 8, wherein said third coating is a coating of titaniumcarbonitride.
 11. The cutting tool insert of claim 10, wherein saidthird coating is 2 to 5 microns thick.
 12. The cutting tool insert ofclaim 8, wherein said metal carbonitride of said third coating has anitrogen content of 70% to 90% based upon the total nitrogen and carboncontent of said metal carbonitride layer.
 13. The cutting tool insert ofclaim 8, wherein said metal carbonitride of said third coating has anitrogen to carbon atomic ratio of 0.75 to 0.95 as determined by x-raydiffraction.
 14. The cutting tool insert of claim 8, wherein: said firstcoating is a coating of hafnium nitride 2 to 5 microns thick; saidsecond coating is a coating of aluminum oxide 1 to 10 microns thick, andsaid third coating is a coating of titanium carbonitride 2 to 4 micronsthick, and said plurality of coatings optionally further includes afourth coating of at least one of titanium nitride and titanium carbide1 to 4 microns thick overlaying and in contact with said second coating.15. The cutting tool insert of claim 14, wherein: said second coating isabout 6 microns thick; said third coating is about 3 microns thick; andsaid optional fourth coating is about 2 microns thick.
 16. The cuttingtool insert of claim 8, wherein said metal carbonitride is of a metalselected from the elements of groups IVB, VB, and VIB of the periodictable.
 17. The cutting tool insert of claim 16, wherein said substratecomprises 3 to 30 weight percent binder and 70 to 97 weight percent of acarbide selected from the group consisting of tungsten carbide, titaniumcarbide, tantalum carbide, niobium carbide, molybdenum carbide,zirconium carbide, and hafnium carbide.
 18. The cutting tool insert ofclaim 17, wherein said substrate further comprises a nitride selectedfrom the group consisting of titanium nitride, tantalum nitride, niobiumnitride, molybdenum nitride, zirconium nitride, and hafnium nitride. 19.The cutting tool insert of claim 17, wherein a surface layer of saidsubstrate is enriched in said binder relative to a remainder of saidsubstrate.
 20. A method of making a cutting tool insert including a hardsubstrate and a plurality of coatings, the method comprising: applying afirst coating of at least 2 microns to at least a portion of thesubstrate, the first coating comprising at least one of a metal carbide,a metal nitride, and a metal carbonitride of a metal selected from thegroup consisting of zirconium and hafnium; and applying a secondcoating, said second coating comprising at least one of a metal carbide,metal nitride, and metal oxide of a metal selected from groups IIIA,IVB, VB, and VIB of the periodic table.
 21. The method of claim 20,wherein said first coating is at least 2 microns up to 5 microns. 22.The method of claim 20, further comprising: applying a third coating,intermediate said first coating and said second coating and in contactwith said second coating, said third coating of a metal carbonitride 2to 6 microns thick.